1. Field of Invention
The invention is related to semiconductor light emitters.
2. Description of Related Art
Throughout this application, references are cited. The respective disclosure of each of these references is incorporated in its entirety by reference.
Light emitting diodes (LEDs) are semiconductor devices that generate light from electrical excitation where electrons and holes combine to annihilate, and thereby forming photons.
These structures are typically grown on sapphire or silicon carbide substrates by OMVPE (Organo-Metalic Vapor Phase Epitaxy).
FIG. 1 shows one example of an OMVPE grown standard group III-nitride semiconductor LED that comprises of a sapphire substrate (101), an intrinsically doped gallium nitride (GaN) buffer layer 2 μm thick (102), a silicon doped 2 μm n-type GaN layer (103), an Indium Gallium Nitride (InGaN) active region (104) comprised of a single quantum well or multiple quantum wells, a current blocking layer (105) comprising of magnesium doped p-type AlGaN, and a magnesium doped p-type GaN layer (106).
This LED structure is epitaxially grown on a substrate, which in this case is sapphire, such that several LEDs are formed on the surface of the substrate and electrical terminals (207) (208) are positioned on the n-type GaN layer (203) and the p-type GaN layer (206) of each single LED as shown in FIG. 2.
Group III-nitride LEDs require a thick GaN buffer layer (102) of about 2 μm when grown on a nonconductive sapphire substrate as described in U.S. Pat. No. 4,855,249 (Isamu Akasaki et al., Aug. 8, 1989) and U.S. Pat. No. 5,686,738 (Theodore D. Moustakas, Nov. 11, 1997). This is to achieve device quality material before the n-type layer (103), active region (104) and p-type layers (105) (106) of the device are grown. Although there are other methods, generally this extended 2 μm thickness is desirable to allow the GaN buffer (102) to coalesce during growth resulting in device quality material.
Standard group III-nitride semiconductor LEDs usually have low light extraction due to the refractive index contrast between the semiconductor (NGaN=2.4) and air (Nair=1). Most of the light emitted inside an LED is unable to escape through Snell's window to reach the outside medium (air), and thus has about 6% extraction efficiency from the extraction surface of the LED.
One method to improve light extraction may involve shaping the light exiting surface of the device to reduce the amount of generated light that is lost to total internal reflection as described in U.S. Pat. No. 5,779,924 (Michael R. Krames et al., Jul. 14, 1998). A shaping technique that improves light extraction comprises random texturing of the device surface to achieve light scattering.
Another light extraction enhancement approach is to form a layer with a photonic crystal structure as described in U.S. Pat. No. 6,831,302 (Alexei A. Erchak et al., Dec. 14, 2004). If designed accordingly, a photonic crystal may inhibit guided modes so that more light is extracted through vertical modes or direct guided modes out of the device by diffraction. Surface texturing and photonic crystal structures suffer from added complexity due to extra techniques and processing steps which may include extra layer formation or etching steps.
As shown in FIG. 3, some III-nitride LEDs utilize a metal mirror contact (308) on the p-type GaN layer (306) side of the device as described in U.S. Pat. No. 6,573,537 (Daniel A. Steigerwald et al., Jun. 3, 2003). The metal mirror contact is deposited after the epitaxial process. In this example the metal mirror contact (308) covers the entire p-GaN layer (306). Adjusting the p-GaN layer thickness and as a result positioning of the mirror allows the device to utilize optical cavity (309) effects as seen in FIG. 3 and FIG. 4. Light emitted from the active region (304) self interferes due to reflection from the closely placed metal mirror (308). Optical cavity effects may increase the light emission into the vertical modes while reducing the total number of horizontal optical modes. In this example, vertical modes are readily extracted through a transparent substrate though Snell's window. Positioning the center of the active region (304) of the LED at, or close to, a maximum (402) of the optical field distribution (401) as shown in FIG. 4, assists in light extraction. The local maximums (402) of the optical field distribution (401) are referred to as antinodes (402) because they are antinodes of the standing optical wave. Generally, positioning the center of the active region (304) at or close to the closest antinodes (402) to the metal contact (308) is desired to allow more light through Snell's window. As shown in FIG. 4, antinodes (402), of the standing optical wave (401), are positioned periodically away from the metal contact (308).
The above mentioned light extraction approaches are more easily applied to the p-type GaN layer side of the device which is readily accessible for further processing after the epitaxial crystal growth is completed. Because the sapphire substrate is nonconductive and it is preferred to grow the conductive n-type GaN layer side first, the n-type GaN layer side becomes buried and difficult to access from the substrate side for further processing due to the extreme hardness of sapphire.
Dual-mirrored resonant cavity light emitting diodes (RCLEDs) or microcavity light emitting diodes (MCLEDs) represent a further method of increasing light extraction from a semiconductor light emitting device. The active region is located within an LED in such a way as to create an optical cavity between two properly placed mirrors that direct light emission into vertical modes or a single mode by reducing the total number of optical modes within an LED.
Coupling a highly reflective mirror with a partially reflective mirror to create a cavity has been predicted to increase light extraction efficiencies in the 30% to 50% range over standard LEDs as mentioned within U.S. Pat. No. 6,969,874 (Gee et al., Nov. 29, 2005).
The key to a properly functioning cavity LED is the placement of the mirrors relative to the active region to obtain resonance and constructive interference.
Coupling a metal mirror on the outer p-type III-nitride surface of a LED to an active region placed at or close to an antinode of a standing optical wave can be performed relatively easily with standard deposition techniques as described in U.S. Pat. No. 6,573,537 (Daniel A. Steigerwald et al., Jun. 3, 2003).
One technique to couple a mirror within the n-type III-nitride region at or close to an antinode of an standing optical wave includes the epitaxial growth of Distributed Bragg Reflectors (DBRs) composed of alternating layers of semiconductor materials, each with different refractive indexes and quarter wavelength thicknesses. A large number of these layers may be required to achieve sufficient reflectivity for the optical cavity.
While these DBRs may be grown epitaxially, they have a number of inherent disadvantages. The alternating material layers often suffer from a lattice mismatch that may lead to increased wafer cracking, poorer crystal quality, reduced yield, lower uniformity, and higher manufacturing cost. Moreover, DBRs are more electrically resistive, compared to metal or other semiconductor materials, resulting in poor current injection into the device.
U.S. Pat. No. 6,969,874 (Gee et al., Nov. 29, 2005) discloses a Flip-Chip Light Emitting Diode with a Resonant Optical Microcavity using a DBR at or close to an antinode of a standing optical wave. The DBR specified for the device uses better lattice matched materials and requires fewer alternating layers compared to previous DBR configurations. Nevertheless, while potentially an improvement over previous DBRs, the device has not adequately solved the manufacturing complexities or conductivity shortcomings inherent in DBR material composition.
Another method to place a mirror within the n-type III-nitride region comprises of removing the base substrate and any buffer layers, followed by a thinning and polishing of the n-type III-nitride layer in such a way as to create an interfacial mirror located optimally for the microcavity.
U.S. Patent Application Publication 2007/0096127 (P. Morgan Pattison, May 3, 2007) discloses a MCLED with an interfacial mirror on the n-type side of a III-nitride device coupled with a metal mirror deposited on the p-type III-nitride side of the device and an enclosed active region placed at or close to an antinode of an standing optical wave between the two reflective surfaces.
Fabrication of this MCLED requires laser-lift-off, as described in U.S. Pat. No. 6,071,795 (Nathan W. Cheung et al., Jun. 6, 2000), to remove the substrate. Additionally the n-type III-nitride layer must be etched to a precise and accurate thickness to create an optimally positioned interfacial mirror relative to the active region and a highly reflective metal mirror deposited on the p-type III-nitride surface of the device.
While this approach has been shown to function, the process of laser-lift-off and subsequent etching is difficult to commercialize and to obtain high yields.
What is desired is a microcavity LED structure that does not require cumbersome material removal or complicated layering. Further it is desirable to combine various light extraction structures as described above while allowing for high current injection.